SOURCES OF E. COLI AT THE LAKE-GROUNDWATER INTERFACE OF BEACHES ALONG THE SHORELINES OF LAKE HURON AND GEORGIAN BAY, ONTARIO, CANADA

Escherichia coli (E. coli) are frequently found in the lake water adjacent to most beaches along the shores of the Great Lakes during the summer, and often at levels than result in beach postings and/or closures to swimming. The presence and distribution of E. coli at the water table were investigated along the groundwater-lake interface and below the adjacent beaches at Amberley Beach on Lake Huron and Woodland Beach on Georgian Bay. Groundwater flow beneath these beaches is affected by infiltration and by fluctuations in lake levels. Infiltration maintains the water table at an elevation higher than the lake throughout the year, and hence groundwater flow is always towards the shoreline. Hence, any bacteria entering the groundwater flow regime and that could move freely with groundwater flow would be transported towards the lake, and would discharge at the shoreline. However, our studies show that E. coli are present in sand and groundwater only below the swash zone and within a few metres adjacent to the swash zone (<15 m from the shoreline); E. coli was not detected in sand or groundwater below the beach or adjacent dunes. Thus, there does not appear to be any groundwater transport of E. coli to the lake from the beach-front residences' septic systems that are located about 50 - 60 m from the shoreline, nor are there any self-sustaining population of E. coli beneath the beach away from the swash zone. Thus, it appears that the source of E. coli in the sand at the lake-groundwater interface is the lake. During storm events, a storm-induced wave runup of up to 10 m horizontally across the swash zone would cause infiltration of lake water, and any E. coli in the lake water, at the swash zone. This would raise the water table beneath the swash zone above the rest of the beach, causing a reversal in the direction of groundwater flow and transport of E. coli into the groundwater regime. Although wave runup storms can cause a water table rise ~100 m from the shoreline, our studies show that the zone actually receiving infiltrated lake water is restricted to less than a few metres from the swash zone. Hence, should E. coli infiltrate into the subsurface during wave run-up, E. coli would be restricted to within a few meters of the swash zone. Therefore, high E. coli numbers that cause beach closures may come from the beaches themselves.